Recording (macOS): pin the keyframe interval and count dropped frames #937

Part of #920 (ranks 17 and 18). Effort S, risk low.

Problem

• The keyframe interval is left to VideoToolbox (ScreenCaptureRecorder.swift sets no AVVideoMaxKeyFrameIntervalKey). The editor's scrub cost is the GOP length; Windows uses 1 s and Linux 0.5 s.
• Frames the writer input refuses (isReadyForMoreMediaData false or append failing) are dropped without a count, so no macOS lever can be measured.

Proposal

• Pin the keyframe interval (1 s, like Windows) with AVVideoMaxKeyFrameIntervalKey and AVVideoMaxKeyFrameIntervalDurationKey.
• Count dropped frames and report them in the helper's stop event.

Verify

• Swift unit test where the logic is pure (CI swift-macos-helper).
• On a Mac: ffprobe -show_entries packet=flags shows a keyframe every second; the stop event reports the drop count.